Abstract

High-power, coherent radiation from semiconductor lasers is attractive for such diverse applications as free-space communication, optical data storage, and microsurgery. However, several factors conspire to prevent near-ideal performance from broad area devices and laser arrays. Waveguides wider than a few microns support many lateral modes with poor gain discrimination. Consequently, such modes are easily "mixed" by perturbations in gain and refractive index caused by gain saturation, thermal gradients, and inhomogeneities that are due to imperfect crystal growth. This causes spatially localized modes, multimode operation, and reduced spatial coherence, all of which lead to farfields broader than the "diffraction limit."

In this thesis, we have investigated the influence of gain saturation on the lateral modes of broad area structures and laser arrays. Analytical and numerical techniques have been developed to solve self-consistently for mode shapes and propagation constants as a function of injected current density above threshold. For example, our analysis indicates that the gain-saturated modes of broad area lasers consist of an integer number of phase-locked "filaments." In gain-guided quantum well lasers, these nonlinear modes are observed to oscillate into narrow, single-lobed farfields, which broaden only slightly with increased power output up to the 500mW level. Conversely, laser arrays have been widely touted as structures that suppress unwanted filamentation in favor of spatial mode control. Indeed, in this work we have demonstrated supermode control at the 100 mW power level by varying the diffraction region length in diffraction-coupled arrays. Both theoretically and experimentally, however, we have found the lateral modes of laser arrays to be unstable with increased current injection. Waveguides that are phase-matched below threshold become detuned under the influence of gain saturation, so that interguide power transfer is reduced. This decreases the injection-locking bandwidth, and ultimately, the spatial coherence. While undesirable for a laser oscillator, this property may be attractive for all-optical switching in nonlinear directional couplers.

Finally, we have considered marrying the high-power, coherent output of broad area lasers and laser arrays with the broadband tunability possible in semiconductor lasers. In particular, the steplike density of states unique to quantum well structures results in gain spectra that are broader and flatter than comparable spectra of double heterostructure lasers. Experimentally, we have tuned uncoated, single quantum well stripe lasers in a grating-coupled external cavity over a range >125 nm centered about 800 nm. Similarly tuned broad area lasers output in excess of 200 mW (pulsed) into a single longitudinal mode over 80 nm, and buried heterostructure lasers were operated continuously over 90 nm. We expect that in the future, such devices could provide a compact, rugged, more efficient alternative to dye lasers.